It is known that paraffinic hydrocarbons boiling in the diesel range may be produced from triglyceride-comprising feedstocks from biological origin such as vegetable oil, animal fat or fish oil.
In U.S. Pat. No. 4,992,605 for example is disclosed a process for producing hydrocarbon products in the diesel boiling range, mainly C15-C18 straight chain paraffins. The process comprises hydroprocessing vegetable oils or some fatty acids at conditions effective to cause hydrogenation, hydrotreating and hydrocracking of the feedstock (temperature 350-450° C.; pressure 4.8-15.2 MPa; liquid hourly space velocity 0.5-5.0 hr−1) using a commercially available hydroprocessing catalyst. Cobalt-molybdenum and nickel-molybdenum hydroprocessing catalysts are mentioned as suitable catalysts. Co—Mo and Ni—Mo catalysts are exemplified in the examples. In the process of U.S. Pat. No. 4,992,605, straight chain paraffins are produced that have undesirable cold flow properties, i.e. a relatively high pour point and cloud point.
In U.S. Pat. No. 5,705,722 is disclosed a process for producing liquid hydrocarbons boiling in the diesel fuel range from a biomass feedstock comprising tall oil with a relatively high content of unsaturated compounds. The feedstock is hydroprocessed at a temperature of at least 350° C. Cobalt-molybdenum and nickel-molybdenum hydroprocessing catalysts are mentioned as suitable catalysts. Co—Mo and Ni—Mo catalysts are exemplified in the examples. Also in the process of U.S. Pat. No. 5,705,722, mainly normal alkanes are produced that have undesirable cold flow properties, i.e. a relatively high pour point and cloud point.
In EP 1 396 531 is disclosed a process for converting a feedstock selected from vegetable oil, animal fats or fish oil into liquid hydrocarbons, the process comprising a hydro-deoxygenation step followed by a hydro-isomerisation step. In this way, branched hydrocarbons with desirable cold flow properties are produced. The hydro-isomerisation step is operated using the counter-current flow principle. It is mentioned that for the hydro-deoxygenation step typically NiMo or CoMo catalyst are used. For the hydro-isomerisation step, the catalyst may comprise Pt, Pd or reduced Ni. Noble metal hydro-isomerisation catalysts (Pt or Pd) are preferred and exemplified. The feedstock is preferably pre-hydrogenated under mild conditions prior to the hydro-deoxygenation step.
In the process of EP 1 396 531, an expensive noble metal catalyst is used in the hydro-isomerisation step. Since noble metal catalysts are very sensitive to catalyst poisons, it is necessary to remove impurities from the effluent of the hydro-deoxygenation step. This is done by counter-current operation of the hydroisomerisation step and/or a stripping step between the hydro-deoxygenation and the hydro-isomerisation step.
In WO2008/058664 a process is disclosed for producing hydrocarbon fractions which can be used as diesel fuel or a component of diesel fuel, starting from a mixture of biological origin containing esters of fatty acids possibly with amounts of free fatty acids. The process disclosed in WO2008/058664 comprises a deoxygenation step with a hydrogenation catalyst, a purification step comprising separating a liquid fraction from step (a) and a washing step, and a hydroisomerisation step. The catalyst exemplified for the hydroisomerisation step comprises platinum on alumina, hence a noble metal catalyst sensitive to catalyst poisoning that requires the purification step.